Is it possible in NAV 2018 to apply a discount to a payment when you are actually posting it. So for example a customer owes €200.20 but pay me €200. HOw to I apply the discount of .20cent there and then when I am posting the payment to their account. Thanks
Yes,take a look at payment tolerances.
You can watch this video
https://usedynamics.com/business-central/finance/fields-in-customer-posting-groups/
It should explain about it - I know it is for Dyn 365 BC but the functionality is exactly the same.
Also this one:
https://www.archerpoint.com/blog/Posts/dynamics-nav-2013-how-use-payment-tolerance-save-you-time
Yes, it is eve possible for NAV 5.0. You can check Docs for the finance part